In large pipelines transporting fuel fluids it is very important to provide safe, reliable and automatic controls in the event of a line break or serious leak, for operating shutoff valves in main lines, by-pass lines, crossover lines, river-crossing lines, off-shore lines, and the like.
Certain prior line break controls utilize a complicated system comprising differential relay valves, diverter valves, check valves and holding tanks, with an adjustable timing relay valve for adjusting the time delay after a substantial pressure drop before the system operates to actuate the valve shutoff control. Such complicated systems are quite expensive.
Certain other conventional line break controls utilize a piston or diaphragm as a pressure responsive element to actuate the valve shutoff control, the main line being connected directly to one side of the pressure responsive element and through a reduced orifice to the other side and to a pressure tank which actuates the element due to a severe drop in the direct connection. Such a control is disclosed in my prior U.S. Pat. No. 2,849,987. These controls rely on a lost motion connection between the diaphragm and the shutoff control to absorb normal pressure fluctuations without actuating the control, and are operated by an amount of pressure drop as related to an estimated rate of pressure change, rather than by an actual rate of pressure drop. A pressure drop sufficient to operate this type of shutoff control may be caused by conditions other than a line break under which it may not be desired to shut off the main valve.
Moreover, this type of shutoff control is not very reliable or accurate because the compressibility of the high pressure gas in the system tends to cause fluctuations of the pressure responsive element which may cause operation of the control lever when not desired.